Misconception. Memory Bandwidth (or video ram) isn't directly limited by Bus.
Xbox only has a 128bit bus with 512MB's of Vram. Yet it it's graphics are gorgeous at times. The 5900 series from nvidia had a 256 bus and was pushing 22-27GB/s. The 8800GTS has a 256bus and pushes 62.2/GBs
While it can affect the bandwidth, they seem to be finding increasingly amazing ways around it. Especially since the change from AGP to PCI-e... If I had to guess, I think thats when bus size became less important.
People believe that the newer cards 9xxx series with a 256bus won't be able to utilize a gig of video ram. While again, that was probably true 3-4 years ago, it's definately nottrue now. Games towards the end of this year and next will use that ram, and your only limiting factor won't be bus width, but memory bandwidth. What the card can push out it's rear end. If a card has a 128bit bus, but can push +100GB/s, thats a card I'd like to own.